Final answer:
Gravity is the key force in star evolution, impacting a star from its formation through to its end state. Massive stars with higher gravity have higher temperatures and luminosity, while lower mass stars are cooler and less bright. The H-R diagram tracks these changes in a star's lifecycle, particularly temperature and luminosity.
Step-by-step explanation:
Gravity is the most significant force that determines the evolution of stars. The self-gravitational energy of a star is what triggers fusion, which leads to the star shining. Throughout a star's lifecycle, gravity plays a crucial role, starting from the initial collapse of a protostar, entering the main sequence, and influencing its ultimate fate, be it as a red giant, supernova, or forming a black hole at the end of its life. Massive stars with more gravity can compress their cores more, leading to higher temperatures and greater energy generation. Consequently, these stars are more luminous and have shorter lifespans. Conversely, stars with lower mass possess less gravity, resulting in cooler interiors and lower luminosity.
The H-R diagram is a useful tool to follow these changes in a star's evolution, showcasing the relationship between a star's temperature, luminosity, and stage in its lifecycle. The evolution of our own Sun, and its impact on Earth, can also be traced with this diagram - as the Sun ages and becomes a red giant, it will eventually shed its outer layers, and the remaining core will collapse, forming a white dwarf. The luminosity and temperature of the Sun and stars like it at various points of their evolution can be precisely mapped on the H-R diagram.